Polymer/polyol compositions suitable for use in producing polyurethane foams, elastomers and the like are known materials. The basic patents in this field are U.S. Pat. Nos. 3,304,273, 3,383,351 and U.S. Pat. No. Re. 28,715 to Stamberger. Such compositions can be produced by polymerizing one or more olefinically unsaturated monomers dissolved or dispersed in a polyol in the presence of a free radical catalyst. These polymer/polyol compositions have the valuable property of imparting to, for example, polyurethane foams and elastomers produced therefrom, higher load-bearing properties than are provided by unmodified polyols.
In addition, U.S. Pat. No. 3,523,093 to Stamberger discloses a method for preparing polyurethanes by reacting a polyisocyanate with a mixture of a polyol solvent medium and a preformed normally solid film-forming polymeric material obtained by polymerization of ethylenically unsaturated monomers. The film-forming polymer may be prepared by various techniques, including polymerizing the monomers in the presence of reactive radical-containing compounds such as alcohols and mercaptans.
The polymer/polyol compositions initially introduced were primarily compositions produced from polyols and acrylonitrile and, to some extent, acrylonitrile-methylmethacrylate mixtures. Such compositions were at least primarily used commercially in producing foams under conditions such that the heat generated during foaming is readily dissipated (e.g.--the foams are of a relatively thin cross-section) or under conditions such that relatively little heat is generated during foaming. When the heat is not readily dissipated, the foams tend to scorch (discolor).
The co-pending Priest application identified hereinabove provides an improved process for forming polymer/polyols from acrylonitrile-styrene monomer systems which includes, in general, maintaining a low monomer concentration throughout the reaction mixture during the process. The novel polymer/polyols produced can be converted to low density, water-blown polyurethane foams having reduced scorch in comparison to all acrylonitrile, and acrylonitrile-methylmethacrylate polymer/polyols. However, the stability of the polymer/polyols decreases with increasing styrene to acrylonitrile ratios. Further, the discoloration (scorch) of the resulting foams still presents some problems, particularly when the polymer composition contains a relatively high acrylonitrile to styrene ratio.
Still further, U.S. Pat. No. 4,104,236 to Simroth discloses additional and substantial improvements in forming polymer/polyols. This allows the optimization of the polymer content and the usable monomer ratios for a given polyol.
The previously identified Shook et al application discloses further improvements in the formation of polymer/polyols. As discussed therein, polymer/polyol compositions exhibiting outstanding properties can be made by utilizing, in the formation of the polymer/polyols, a specific type of peroxide catalyst, namely t-alkyl peroxyester catalysts. By the utilization of this specific type of catalyst, polymer/polyols can be produced on a commercial basis with outstanding properties such as filterability in processing yet which allows either the polymer or the styrene content to be increased. Also, polymer/polyols can be produced on a commercial scale with polyols having a molecular weight lower than have been used prior to this invention.
Despite these improvements, there is still room for further refinement. Thus, in the slabstock foam area, the problem of scorch presents a barrier to the use of acrylonitrile-containing polymer/polyols where the buns have a relatively large cross-section. It would be desirable to, in effect, be capable of providing acrylonitrile copolymer polymer/polyols that would be sufficiently low in acrylonitrile content to provide reliable assurance that the resulting buns would be even less subject to scorch. Achievement of this objective requires the utilization of relatively high levels of styrene or other comonomers, so that the acrylonitrile content is about 30 to 40 percent of the monomer system used or even lower. While such polymer/polyols can be produced with certain limitations by prior techniques, the production is not as commercially trouble-free as is desired.
More particularly, the production of polymer/polyols on a large commercial scale with the economy needed places practical limitations on the minimum ratio of acrylonitrile to styrene or other comonomer used in the monomer system, the minimum polyol molecular weight and the maximum polymer content when prior techniques are employed. Commercial production thus requires that the resulting polymer/polyols have relatively low viscosities so that processing in the production equipment can be economically carried out. Further, the stability resulting must be sufficient to allow operation without plugging or fouling of the reactors as well as allowing for relatively long term storage.
The polymer/polyols must also be capable of being processed in the sophisticated foam equipment presently being used. Typically, the prime requirement is that the polymer/polyols possess sufficiently small particles so that filters, pumps and the like do not become plugged or fouled in relatively short periods of time.
While somewhat simplified, the commercial processability of a particular polymer/polyol comes down to its viscosity and stability against phase separation. Lower viscosities are of substantial practical and economic significance due to the ease of pumping and metering as well as ease of mixing during the formation of polyurethanes. Stability is of prime consideration in insuring that the polymer/polyols can be processed in commercial production equipment without the necessity of additional mixing to insure homogeneity.
It has been recognized that the stability of polymer/polyols requires the presence of a minor amount of a graft or addition copolymer which is formed in situ from the polymer and polyol.
With regard to graft copolymer stabilizers, a number of literature references have observed large differences in grafting efficiency between the use of peroxides such as benzoyl peroxide and azobis-isobutyronitrile in certain monomer-polymer systems. In general, the conceptual thrust is that the use of peroxide catalysts should improve the stability inasmuch as this type of catalyst produces a relatively greater amount of the graft specie.
Others have noted no marked differences in grafting efficiency. In the Journal of Cellular Plastics, March, 1966, entitled "Polymer/Polyols; A New Class of Polyurethane Intermediates" by Kuryla et al., there is reported a series of precipitation experiments run to determine any marked differences in the polymer/polyols produced by either benzoyl peroxide or azobis-isobutyronitrile when used as the initiators in the in situ polymerization of acrylonitrile in a propylene oxide triol having a theoretical number average molecular weight of about 3000. The data indicated no significant differences between the polymers isolated, and no marked "initiator effect" was observed.
With regard to addition copolymer stabilizers, efforts in the polymer/polyol field have been concerned with the incorporation of additional amounts of unsaturation to that inherently present in the polyoxyalkylene polyols typically used in forming polymer/polyols. U.S. Pat. Nos. 3,652,639 and 3,823,201 and Great Britain Pat. No. 1,126,025 all utilize this approach.
None of the above patents recognize the utility of adding a tailored, preformed stabilizer in producing polymer/polyols.
In general, a substantial amount of additional effort has been directed towards dispersion polymerization in organic liquids. This involves the polymerization of a monomer dissolved in organic liquid to produce insoluble polymer dispersed in the liquid as a continuous phase in the presence of an amphipathic graft or block copolymer as the dispersant (stabilizer). According to a recent text, "Dispersion Polymerization in Organic Media", edited by K. E. J. Barrett, John Wiley & Sons, copyright 1975, the development of techniques for the preparation of dispersions of polymers of controlled particle size in organic liquids has been largely motivated by the requirements of the surface coatings industry. The function of the dispersant or stabilizer in a sterically-stabilized colloidal dispersion is to provide a layer of material solvated by the dispersion medium on each particle surface. Every particle is thus surrounded by a tenuous cloud of freely-moving polymer chains which is, in effect, in solution in a continuous phase. This layer prevents the particles from coming into direct contact and also insures that, at the distance of closest approach of the two particles, the attraction between them is so small that thermal energy renders contact reversible.
The most successful type of dispersant devised for use in dispersion polymerization, according to Barrett, has been based on a block or graft copolymer which consists of two essential polymeric components--one soluble and one insoluble in the continuous phase. The dispersant may be either preformed or formed in situ. When formed in situ, a "precursor" is used, i.e., a soluble polymeric component that is introduced into the organic liquid serving as the polymerization medium. The monomer system being polymerized will react with the soluble polymeric component during polymerization to form, in situ, a graft or addition copolymer dispersant. When an addition copolymer dispersant is to be produced, the source of the soluble polymeric component is unsaturated and is termed a "macromonomer". The main requirement for what is termed the "anchor" portion is that it be insoluble in the dispersion medium, but its effectiveness may be greatly enhanced if it has some specific affinity for the dispersed polymer. The criterion of insolubility of the anchor portion also defines, in practice, the minimum size of the soluble portion. For a polymer to be sufficiently insoluble in the dispersion medium, the molecular weight usually has to be of the order of 1000 or greater. The soluble chain attached to such an anchor portion must be at least of similar molecular weight, otherwise a stable micellar solution of dispersant cannot be formed in the dispersion medium; and precipitation occurs. The minimum molecular weight of the soluble component must therefore be at least 500 to 1000, which is considerably greater than the minimum requirement for an effective steric barrier.
Based upon this technology, a large number of patents have been issued. The Barrett text lists some 200 issued United States and foreign patents. Yet, despite this considerable body of technology, applicants are unaware of any attempts prior to the present invention to prepare polymer/polyols by employing preformed stabilizers. Indeed, the prior efforts in dispersion polymerization have been directed to the use of organic liquids as dispersion mediums which have extremely low viscosities, e.g.--typically no more than a few centipoises at 25.degree. C. The theoretical considerations discussed in Napper, Journal of Colloid and Interface Science, 32 pages 106-114 (1970), may well account for the fact that preformed stabilizers have not been used heretofore to stabilize polymers/polyols, despite the recognition that the stability of polymer/polyols requires the presence of a graft or addition copolymer which is formed in situ in conventional polymer/polyol from the polymer and polyol. Thus, the Napper article leads to the conclusion that stabilization would not be effective if the solvatable portion has a chemical composition identical to the polymerization medium.